1. Field of the Invention
The present invention relates to a semiconductor device having a nitride semiconductor layer formed on a gallium nitride (GaN) substrate, and to a method for manufacturing the semiconductor device.
2. Description of the Background Art
Nitride semiconductors, such as gallium nitride, are utilized and studied for light emitting devices and other electronic devices, and blue light emitting diodes and green light emitting diodes making use of the properties of nitride semiconductors are already in practical use. Also, blue-violet semiconductor lasers employing nitride semiconductors are being developed as next-generation high-density optical disk light sources.
Conventionally, the fabrication of light emitting devices with nitride semiconductors mainly uses sapphire substrates. However, the lattice mismatch between the sapphire substrates and nitride semiconductors formed thereon is as large as about 13%, so that a high density of defects, such as dislocations, are produced in the nitride semiconductors, making it difficult to obtain nitride semiconductors of high quality.
Recently, gallium nitride substrates (hereinafter referred to as GaN substrates) with low defect density are developed, and studies and developments are being intensively made for applications of the GaN substrates. The GaN substrates are chiefly intended for use as semiconductor laser substrates.
When a nitride semiconductor is grown on a GaN substrate, there is a problem that good crystallinity is not obtained when the nitride semiconductor is grown on a C-plane, i.e., on a (0001) surface. To solve this problem, Japanese Patent Application Laid-Open No. 2000-223743 discloses a technique for improving the crystallinity of, and lengthening the life of, a nitride semiconductor light emitting device formed on a GaN substrate, in which the upper surface of the GaN substrate is inclined at an angle of not less than 0.03° nor more than 10° with respect to the C-plane.
Techniques for manufacturing semiconductor devices using nitride semiconductors are also disclosed in Japanese Patent Application Laid-Open Nos. 2000-82676 and 2003-60318, for example.
Now, in the fabrication of a semiconductor device such as a semiconductor laser using a GaN substrate, it is desired that the nitride semiconductor layers formed on the GaN substrate present not only good crystallinity but also superior surface flatness. However, when nitride semiconductor layers are grown on a GaN substrate by utilizing the technique of Japanese Patent Application Laid-Open No. 2000-223743, the nitride semiconductor layers may exhibit unsatisfactory surface flatness, and may even fail to ensure sufficient crystallinity. This causes the semiconductor device fabricated with the nitride semiconductor layers to exhibit deteriorated electric characteristics and reduced reliability. Also, it is not preferable to greatly incline the upper surface of the GaN substrate from the viewpoint of manufacture of the semiconductor devices.